Advertising on connected TV like Hulu and Roku is expected to surge nearly 40 percent in 2019 to just under $7 billion, according to eMarketer.

And by 2021, eMarketer expects connected TV advertising to surpass $10 billion. Of course, it’s worth noting this is still just a fraction of the $70 billion TV ad marketplace.

YouTube, Hulu and Roku are leading the market, with users of these platforms either never having subscribed to cable or satellite or are now opting for slimmed-down packages, says Eric Haggstrom, forecasting analyst at eMarketer.  

As a result, advertisers are eager to reach these so-called light TV viewers.

These services also offer the ability for marketers to more precisely and efficiently target consumers, something that historically attracted advertisers to digital.

But there are still some hurdles holding marketers back from shifting their linear TV spend to connected TV, including measurement.

“There is no single, commonly accepted measurement across platforms like there is in TV,” Haggstrom writes. “Also, CTV targeting, attribution and programmatic capabilities are significantly behind those of other leading digital ad platforms.”

And the entrance of several new streaming services that are not ad-supported, like Disney Plus, Apple TV Plus and HBO Max, is expected to make platforms with ads less attractive to consumers, Haggstrom says.

Connected TV users are expected to grow 5.3 percent to 195.1 million in the U.S. by the end of the year and surpass 200 million in 2020.

Roku is the leader with 44.2 percent of viewers, followed by Amazon Fire TV, Google Chromecast and Apple TV.